PURPOSE: To provide a treating furnace for reforming a reduced slag to a composition close to oxidized slag by melting and reacting reduced slag with dust obtained as a steel mill discharge, capable of reducing the size of the furnace body and enabling efficient treatment of reduced slag in powdery state as well as molten state and to provide a process for the operation of the furnace.

CONSTITUTION: The furnace body 12 of a treating furnace 10 is laid in horizontal state and made to be rotatable. A pure oxygen burner 24 is attached to either one of the right and left side-walls and a slag charging port 32 is opened on the other side-wall. A reservoir 16 and a slag discharging port 20 are formed on the bottom of the furnace body. Powdery dust 58 and reduced slag 60 are blasted together with a flame from the pure oxygen burner 24 while keeping the furnace body 12 in horizontal posture. The dust and slag are melted and reacted with each other and the obtained reacted slag is discharged from the slag discharging port 20. As an alternative method, the furnace body 12 is rotated counterclockwise and a large amount of molten reduced slag is charged at a stroke into the furnace body through the slag charging port 32 while rotating the furnace body.